Identify Terminal Pins of A Relay Without Reference to Datasheet

A relay is an electrically activated switch. It consists of an internal coil which creates a magnetic field that attracts a movable lever and then changes switch contacts when a current flows through it. The typical usage of relay is to allow a low DC voltage circuit (circuit #1) to switch on or off a high voltage (DC or AC) circuit (circuit #2) without direct electrical connection between them. This means circuit #1 and circuit #2 are magnetically and mechanically linked but not electrically connected. There are some designations normally used in describing a relay based on its internal structure and pins distribution as shown in the following:

i) Single Pole Single Throw (SPST) – Such relay has 4 terminal pins which consists of a pair of coil pins and a pair of pins which can be made connected or disconnected by activating or deactivating the relay. Such relay can categorized into normally open or normally closed type.

ii) Single Pole Double Throw (SPST) – Such relay has 5 terminal pins which consists of a pair of coil pins, a common pin, a normally open (NO) pin and a normally closed (NC) pin. When the relay is not activated, the common pin is in contact with the NC pin and when it is activated, the common pin will break away from contact with the NC pin and subsequently makes contact with the NO pin. Also, when the relay is deactivated (from activated state), the common pin will conversely break away from contact with the NO pin and return back in contact with the NC pin.

iii) Double Pole Single Throw (DPST) – Such relay has 6 terminal pins which consist of a pair of coil pins and two pairs of pins, where pins in each pair can be made connected or disconnected by activating or deactivating the relay. Such relay is actually a combination of two SPST relay structures with only one coil pin pair.

iv) Double Pole Double Throw (DPDT) – Such relay has 8 terminal pins which consist of a pair of coil pins, two 3 pins group for each group consists of a common pin, normally open (NO) pin and normally (NC) pin. Such relay is actually a combination of two SPDT relay structures with only one coil pin pair.

Figure 1: Various normal designations of relay

Relays that are normally used in electronic circuits are SPDT and DPDT types due to their flexibility in controlling circuit switching. In the next section, a demonstration on identifying the pins of a relay from SPDT or DPDT type without referring to its datasheet will be illustrated. A 5-pins type relay, a 6-pins type relay and an 8-pins type relay will be used in this demonstration.

Hardware required:

Software required:

None

Related Reference:

Part 1: 5 pins relay (SRD-05VDC-SL-C)

1. Let’s start with determining the relay coil pins using a multimeter. Set the multimeter to resistance measuring mode with a scale of 1000 ohm since the coil resistance normally ranges between 50 ohm and 1000 ohm.

2. Turn back the relay to see pins located at its bottom part. Try to touch the probes of multimeter on a pair of pins of the relay until the touched pair shows a resistance value (but not zero).

**Only the relay coil pin pairs will show non-zero resistance value.

**Other pin pairs will show either zero resistance or infinite resistance.

**There is only 1 pair of coil pins found in this case

a) Top view b ) Bottom view

Figure 2: Overview of the relay (SRD-05VDC-SL-C)

Figure 3: Coil pins of the relay is determined (in this case, the coil resistance is 70 ohm)

3. Since the coil pin pair is found. There is only 3 remaining pins to determine which are the NC pin, NO pin and common pin. Set your multimeter to continuity test mode. Try to touch the probes of multimeter on remaining pins to determine which are the pins actually connected with each other.These pins actually consist of normally closed (NC) pin/pins and common pin/pins. Based on this finding, the remaining pins must be normally open (NO) pins.

**In this case, there is only 1 pair of pins found connected. Hence, the 3rd pin must be the normally open (NO) pin.

**Since there is only 1 NO pin, there will be only 1 NC pin in the connected pair. The other 1 remaining pin in the connected pair must be a common pin.

4. To differentiate NC pin and common pin from the connected pins, there is a need to apply voltage across the relay coil so as to activate it. The required activation voltage for the relay can be determined from its product code in the section of code enclosed with a dash which contains a number suffixed with VDC as shown below.

Figure 4: The part of product code indicates relay activation voltage

**05VDC indicates that the activation voltage is DC 5V.

**Then, hinging on this basis, 03VDC would indicate that the activation voltage is DC 3V, 12VDC means an activation voltage of DC 12V and so on.

**More often that not, the activation voltage relay may be less than what is stated in the product code during the real test as long the voltage applied is sufficient to establish a magnetic field that is able to attract movable lever to normally open (NO) pin so that a connection between a normally open (NO) pin and common pin is established.

**Application of the stated activation voltage to a relay can makes it less sensitive to voltage fluctuation especially in power control due to the power or voltage dip phenomenon.

5. Turn on the DC power supply. Without connecting its output terminal to coil pins of the relay, please turn the voltage adjusting knobs until the voltage reading reaches the zero value.

Figure 5: Zero-ing the output voltage of power supply

6. Then, connect the output terminal of DC power supply across the coil pins of the relay using crocodile clips

**Some relays may have internal suppressing diode installed across its coils for protection purpose. Normally, for this kind of relays, the location of the coil pins will be marked as well as their polarity to prevent any mistaken connection that could destroy the relays that have been made.

**In this part, the coil pins of the relay (SRD-05VDC-SL-C) have no polarity marked on them since the internal suppressing diode is not available. Hence, the positive output of DC power supply can be connected to any one of the coil pins while negative output of DC power supply will be connected to another pin of the coil or vice versa.

7. Next, increase the output voltage by slowly turning the voltage adjusting coarse knob only in clockwise direction until a “click” sound emitted by the relay is heard ( this means that the relay is activated). In this case, although the activation voltage stated is DC 5V, the activation voltage recorded is about 3.3V which is lower.

a) Connection of crocodile clips at the relay coil pins b) Connection of crocodile clips at the DC power supply

Figure 6: Connect the output terminal of power supply across the coil pins of relay using crocodile clips

Figure 7: Activation voltage of the relay during this demonstration

8. Then, with the multimeter set to continuity test mode, check which pin is now connected to the normally open (NO) pin in this moment. That pin would be the common pin.

Figure 8: Determine the common pin of the relay when the relay is activated

9. Since the common pin is determined, thus another one remaining pin in the connected pins detected in step 3 must be the normally closed (NC) pin.

10. Now, all the pins of relay have been determined as in Figure 9 below.

Figure 9: The distribution of terminal pins of relay (SRD-05VDC-SL-C) as determined

Part 2: 6 pins relay (TRB-12VDC-SB-CL)

1. Same as previous parts, let’s start with determining the relay coil pins using a multimeter. Set the multimeter to resistance measuring mode with a scale of 1000 ohm since the coil resistance normally ranges between 50 ohm and 1000 ohm.

2. Turn back the relay to see pins located at its bottom part. Try to touch the probes of multimeter on a pair of pins of relay until the touched pair shows a resistance value (but not zero).

**Only the relay coil pin pairs will show non-zero resistance value.

**Other pin pairs will show either zero resistance or infinite resistance.

**There is only 1 pair of coil pins found in this case

a) Top view b ) Bottom view

Figure 10: Overview of the relay (TRB-12VDC-SB-CL)

Figure 11: Coil pins of the relay is determined (in this case, the coil resistance is 720 ohm)

3. Since the coil pin pair is found. There is only 4 remaining pins to determine which are the NC pin/pins, NO pin/pins and common pin/pins. Set your multimeter to continuity test mode.Try to touch the probes of multimeter on remaining pins to determine which are the pins actually connected to each other.These pins actually consist of normally closed (NC) pin/pins and common pin/pins. Based on this finding, the remaining pins must be normally open (NO) pins.

**In this case, 3 pins are found connected. Hence, the 4th pin must be the normally open (NO) pin.

**Since there is only 1 NO pin, there will be only 1 NC pin in the connected pins.Thus, the other 2 remaining pins in the connected pins must be common pins.

4. To differentiate NC pins and common pins from the connected pins, there is a need to apply voltage across the relay coil so as to activate it. The required activation voltage for the relay can be determined from its product code.

5. Turn on the DC power supply. Without connecting its output terminal to coil pins of the relay, please turn the voltage adjusting knobs until the voltage reading reaches the zero value.

6. Then, connect the output terminal of the DC power supply across the coil pins of the relay using crocodile clips

**In this part, the coil pins of the relay (TRB-12VDC-SB-CL) have no polarity marked on them since internal suppressing diode is not available. Hence, the positive output of the DC power supply can be connected to any one of the coil pins while negative output of DC power supply will be connected to another pin of coil or vice versa.

7. Next, increase the output voltage by slowly turning the voltage adjusting coarse knob only in clockwise direction until a “click” sound emitted by the relay is heard (means that the relay is activated). In this case, although the activation voltage stated is DC 12V, the activation voltage recorded is about 7.5V which is lower.

a) Connection of crocodile clips at the relay coil pins b) Connection of crocodile clips at the DC power supply

Figure 13: Connect the output terminal of power supply across the coil pins of relay using crocodile clips

Figure 14: Activation voltage of the relay during this demonstration

8. Then, with the multimeter set to continuity test mode, check which pins now are connected to the normally open (NO) pin in this moment. These pins would be the common pins.

Figure 15: Determine the common pin of the relay when the relay is activated

9. Since the common pins are determined, thus another one remaining pins in the connected pins detected in step 3 must be the normally closed (NC) pins.

10. Now, all the pins of relay have been determined as shown in Figure 16 below.

Figure 16: The distribution of terminal pins of relay (TRB-12VDC-SB-CL) as determined

**The relay (TRB-12VDC-SB-CL) is also a single pole double throw (SPDT) type relay.

Part 3: 8 pins relay (SRC-05VDC-SH)

1. Same as previous parts, let’s start with determining the relay coil pins using a multimeter. Set the multimeter to resistance measuring mode with a scale of 1000 ohm since the coil resistance normally ranges between 50 ohm and 1000 ohm.

2. Turn back the relay to see pins located at its bottom part. Try to touch the probes of multimeter on a pair of pins of relay until the touched pair shows a resistance value (but not zero).

**Only the relay coil pin pairs will show non-zero resistance value.

**Other pin pairs will show either zero resistance or infinite resistance.

**There is only 1 pair of coil pins found in this case

a) Top view b ) Bottom view

Figure 17: Overview of the relay (SRC-05VDC-SH)

Figure 18: Coil pins of the relay is determined (in this case, the coil resistance is 127 ohm)

3. Since the coil pin pair is found. there are only 6 remaining pins to determine which are the NC pin/pins, NO pin/pins and common pin/pins. Set your multimeter to continuity test mode.Try to touch the probes of multimeter on remaining pins to determine which are the pins actually connected to each other.These pins actually consist of normally closed (NC) pin/pins and common pin/pins. Based on this finding, the remaining pins must be normally open (NO) pins.

**In this case, two pin pairs are found connected. Hence, the 5th and 6th pins must be the normally open (NO) pins.

**Since there are 2 NO pins, there will be 1 NC pin in each connected pin pair.Thus, the other one remaining pin in each connected pin pair must be a common pin.

4. To differentiate NC pins and common pins from in the connected pins. there is a need to apply voltage across the relay coil so as to activate it. The required activation voltage for the relay can be determined from its product code.

5. Turn on the DC power supply. Without connecting its output terminal to coil pins of the relay, please turn the voltage adjusting knobs until the voltage reading reaches the zero value.

6. Then, connect the output terminal of DC power supply across the coil pins of the relay using crocodile clips.

**In this part, the coil pins of the relay (SRC-05VDC-SH) have no polarity marked on them since internal suppressing diode is not available. Hence, the positive output of the DC power supply can be connected to any one of the coil pins while the negative output of DC power supply will be connected to another pin of coil or vice versa.

7. Next, increase the output voltage by slowly turning the voltage adjusting coarse knob only in clockwise direction until a “click” sound emitted by the relay is heard (means that the relay is activated). In this case, although the activation voltage stated is DC 12V, the activation voltage recorded is about 3.4V which is lower.

a) Connection of crocodile clips at the relay coil pins b) Connection of crocodile clips at the DC power supply

Figure 20: Connect the output terminal of power supply across the coil pins of relay using crocodile clips

Figure 21: Activation voltage of the relay during this demonstration

8. Then, with the multimeter set to continuity test mode, check which pin in each connected pin pair detected in step 3 now are connected with a particular normally open (NO) pin at this moment (since 2 normally open (NO) pins are found in step 3). These pins would be the common pins.

Figure 22: Determine the common pin of the relay when the relay is activated

9. Since the common pins are determined, thus another one remaining pin in the connected pins detected in step 3 must be the normally closed (NC) pin.

10. Now, all the pins of relay have been determined as shown in Figure 23 below.

Figure 23: The distribution of terminal pins of relay (SRC-05VDC-SH) as determined

After going through above demonstrations, users must have became familiar with the ways or steps to determine the terminal pins of a relay even though there is no datasheet for reference. That’s all from for this tutorial, see you next time!!

There is a 4-channel relay module. It has 6 pins: IN1, IN2, IN3, IN4, Vcc and GND. So, i connected them to Arduino UNO pins and only 1 relay worked. Vcc=4,5V, Icc=69mA. Other pins approximately also at this values. Does Icc needs more current for 4-channel? I’m gonna to connect 6 pcs of 4-channel to Arduino Mega 2560 R3.

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